Contrasting microbial carbon transformation pathways drive differential SOC sequestration in long-term biochar-amended paddy and upland soils

Abstract The sustained application of straw-derived biochar (BC) demonstrates considerable potential for enhancing soil organic carbon (SOC) sequestration in agricultural systems, though its efficacy is strongly influenced by soil properties, climate, and other environmental factors. We conducted an 11-year outdoor column experiment involving BC application (11.3 t ha −1 ) under strictly controlled climatic and water–fertilizer conditions to examine the effects of successive seasonal BC application on SOC sequestration across three soil parent materials (Quaternary red clay, Tertiary red sandstone, and Yellow River alluvium) and two land-use types (paddy and upland) in China. Results showed that the joint effect of land use and parent material determines SOC sequestration efficiency of BC. Paddy soils exhibited significantly greater SOC storage, 66–323% higher than that in upland soils under identical parent material conditions. In paddy soils, recalcitrant carbon contributed a larger proportion to SOC changes than in upland soils following long-term BC application. Land use modulated microbial responses to BC: paddy soils showed higher ratios of Gram-positive to Gram-negative bacteria (G + /G − ) and lower fungal-to-bacterial ratios (F/B), whereas upland soils displayed the opposite trend, particularly under Quaternary red clay and Tertiary red sandstone. G + /G − and F/B ratios correlated positively with O-alkyl and alkyl carbon and negatively with aromatic carbon, underscoring their critical role in shaping SOC composition. Soil parent material markedly influenced microbial necromass carbon accumulation under BC amendment. Specifically, Quaternary red clay and Yellow River alluvium enhanced microbial necromass accumulation, particularly fungal-derived carbon, indicating that acidic clay loam and slightly alkaline silt loam soils are more conducive to long-term SOC stabilization. This study provides valuable insights for optimizing site-specific BC application strategies to enhance SOC sequestration. Graphical Abstract